Study on intrinsic properties of organic semiconductors appeared under ultrahigh vacuum

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The mechanism of carrier transport in organic semiconductors and carrier injection from metal electrodes becomes the most important subject to be elucidated for the construction of high performance organic thin film devices. The electrical properties are modified easily by adsorbed gas molecules, which makes it difficult to obtain reliable experimental results. We have studied intrinsic properties under ultrahigh vacuum conditions to strip away the veil of oxygen. We investigated electrical properties of titanyl-phthalocy anine (TiOPc) films under ultrahigh vacuum (UHV) conditions to avoid the influence of gas adsorption. The field-effect measurement revealed that TiOPc films exhibited an n-type semiconducting behavior in UHV. The electron mobility at room temperature was 9 × 10^<-6> cm^2V^<-1>S^<-1> with activation energy of 0.20 eV. The conductivity and carrier density were 9 x 10^<-8> Scm^<-1> and 6 x 10^<16> cm^<-3>, respectively. A clear conversion from n-type to p-type behavior was observed when the films was exposed to oxygen. Strict control of a tmosphere made it possible to obtain a quasi-intrinsic state where both p- and n-type conductions appeared simultaneously.